The present invention is directed to thermoforming and, more particularly, to a continuous process and apparatus for thermoforming polyesters into articles such as ovenable containers, food packaging trays, and the like.
Continuous vacuum-forming devices for making containers from thermoplastic sheets typically utilize a continuous sheet of molten plastic which is extruded and vacuum-formed on a continuous belt or a rotating drum having a plurality of mold cavities. Many of these devices utilize residual heat from the extrusion process, thus avoiding the need to reheat the plastic sheet prior to thermoforming. It generally is considered desirable that heat-set articles such as ovenable containers have relatively uniform thermal crystallinity throughout, the article to provide adequate dimensional stability and impact resistance. For example, Demerest U.S. Pat. No. 5,614,228 describes a continuous rotary thermoforming apparatus in which a sheet of molten polyethylene terephthalate is extruded and vacuum-formed on a rotating drum having a plurality of mold cavities around its circumference. A hot oil circuit and electric heating elements are provided to impart different amounts of heat to different locations of the sheet during thermoforming. According to Demerest, additional heat is applied to portions of the articles that have a greater wall thickness to produce more uniform crystallinity throughout the article. The sheet is required to be tensioned and oriented during article forming to prevent the sheet from warping or otherwise distorting during cooling. Orienting the sheet also is said to result in articles having high impact resistance.
Several drawbacks exist with the type of thermoforming device described by Demerest. For example, a minimum amount of crystallinity, which is stated to be at least about 20%, must be obtained in the article to permit the article to be removed from the mold cavity without significant distortion. Thus, the device is not useful for applications where lower degrees of crystallinity may be desired in an article or a portion thereof. Moreover, the degrees of crystallinity actually obtained by using the Demerest apparatus typically are significantly higher than the stated minimum degree, and cannot be controlled effectively. Another drawback is that forming the sheet under tension results in distortion of the article after molding, which limits the ability of the apparatus to be used for many applications requiring especially high tolerances.
The device described in Demerest also is limited in terms of production speed. Following thermoforming, the articles undergo a series of cooling and drying steps prior to being separated from the mold cavities. The formed sheets are (again) tensioned to prevent distortion during separation from the mold. This type of procedure places severe limitations on production rates, especially for larger sized articles.
Dalgewicz U.S. Pat. No. 6,077,904 discloses a thermoforming process for preparing polyesters that are said to have improved impact properties, low oxygen permeability, and low dimensional shrinkage during heating. According to Dalgewicz ""904, impact modifiers are dissolved into molten polyester to form a eutectic alloy. On slow cooling, the eutectic alloy is said to freeze to form a mixture of particles of the impact modifier embedded in a matrix of the polyester. By controlling the solidification of the melt, it is said that the size and distribution of precipitates of impact modifier from the melt can be controlled to permit control of the mechanical properties of the composition.
Dalgewicz ""904 suffers from several drawbacks. For one, the thermoformed polyesters are extremely brittle, limiting their usefulness in many applications. The eutectic alloy formed requires the use of polyesters having a high initial intrinsic viscosity (I.V.), and also makes the polyesters more susceptible to thermal gradients upon the slow cooling. In addition, large 3D spheroids are developed in the polyesters, resulting in a high 3D morphology, which is undesirable in many applications. Yet another disadvantage of Dalgewicz ""904 is that the required cooling rate is very slow. Slow cooling increases the overall time required for processing, which reduces efficiency and cost effectiveness.
Manlove U.S. Pat. No. 6,086,800 teaches a process and apparatus for continuously thermoforming articles. The apparatus has a plurality of mold facets, each of which has (i) a static upper mold facet section and (ii) a dynamic lower mold facet section to which a mold cavity is attached. The two-part mold facet defines a relatively deep mold, i.e., adapted to form deep-drawn articles. A thermoplastic sheet covers each mold facet and is held in place by a vacuum groove located on the upper mold facet section. The material is shaped by actuating an assist plug in combination with a controlled evacuation of air from the mold cavity.
Manlove also suffers from numerous drawbacks. For example, the static upper mold facet section is situated above the mold cavity. This means that the material is formed over the static upper mold facet and then into the lower mold cavity, resulting in poor mold definition. Also, the static upper and dynamic lower mold facet configuration substantially limits production speed and increases waste, i.e., results in larger amounts of unused xe2x80x9ctrimxe2x80x9d that must be discarded or recycled. Further, the lack of proximity of the upper mold facet section relative to the lower mold facet section prevents the upper mold facet section from being an effective means for influencing the temperature of the thermoplastic material within the mold cavity, in particular the portion that is formed into the article. The mold facet configuration also encounters alignment difficulties at high temperatures due to thermal expansion, which effectively limits the device to low temperature applications.
Gartland U.S. Pat. No. 4,469,270 describes a discontinuous thermoforming apparatus having a mold for thermoforming a plastic article having a flange portion. Vacuum and/or pressurized gas is used to conform a sheet to the shape of the heated mold. External cooling means are provided to maintain a portion of the flange of the article at a temperature that is said to be insufficient to induce undesirable thermal crystallization. This portion of the article preferably has a degree of crystallinity of not more than 10% to improve adhesion of lidding films to the article. The remaining portions of the flange and the remainder of the article are said to preferably have the same average crystallinity.
It would be desirable to develop a continuous process and apparatus for thermoforming articles having excellent heat resistance and dimensional stability. It also would be desirable to develop a continuous process and apparatus for thermoforming articles that exhibit excellent stress relaxation and that do, not undergo appreciable distortion during cooling. It would be especially desirable to develop a process and apparatus capable of faster production times while substantially avoiding distortion, even for the production of larger sized articles.
According to one embodiment of the present invention, a continuous process for preparing a thermoformed article comprises extruding a thermoplastic layer through an extrusion die to form an extrudate in a substantially non-oriented state. The extrudate is contacted with a mold surface, such as a mold cavity (female mold) or a male mold. A stripper plate is disposed adjacent to the mold surface for controlling the temperature of proximate areas of the extrudate, e.g., the area that is formed into the flange portion of a container. The stripper plate optionally is also used for assisting in separating the articles from the mold surface by lowering the entire mold relative to the stripper plate. The extrudate remains in contact with the mold surface for a time sufficient to form the article. Ovenable containers and other articles requiring high temperature resistance typically are heat-set.
The temperature of the mold surface is controlled to maintain the extrudate in a thermoformable state. The mold surface temperature or temperature gradient is controllably selected to induce a predetermined degree of crystallinity or a predetermined crystallinity gradient in the article. The temperature of the mold surface thus is dependent on the physical and chemical properties of the thermoplastic material(s) used, as well as the desired properties of the final article. The stripper plate is maintained at a different (usually lower) temperature than the mold surface to control thermally induced crystallinity in proximate portions of the article. The temperatures of the mold and the stripper plate are suitably selected to achieve stress relaxation in the article, which permits the article to be separated from the mold without or substantially without distortion, independent of the level of thermally induced crystallinity.
In an alternative embodiment of the present invention, a multi-layered article is formed in a continuous process by co-extruding at least two distinct thermoplastic materials, followed by thermoformning under the conditions as described above. In one preferred embodiment, the co-extrudate comprises a polar thermoplastic layer, an intermediate tie layer, and a non-polar thermoplastic layer. The polar layer (e.g., PET) can form the external surface of the article, while the non-polar layer (e.g., polyethylene) can form the internal surface of the article, for example to provide improved sealing properties with various lidding materials, especially using gas flushed sealing or modified atmosphere packaging (MAP). The present invention also is directed to a continuous apparatus for thermoforming multi-layered articles.
According to another aspect of the present invention, a thermoplastic polymeric composition comprises an alkylene terephthalate or naphthalate bulk polymer, an additive, and a compatibilizer/emulsifier/surfactant (CES). The additive comprises a substantially amorphous co-polymer of ethylene and an acrylate. The CES comprises a grafted or backbone co-polymer or ter-polymer of ethylene and a glycidyl acrylate, maleic anhydride, or mixture thereof, and optionally an acrylate selected from the group consisting of methacrylate, ethylacrylate, propylacrylate, butylacrylate, ethylhexylacrylate, and mixtures thereof. When the composition is heat set and formed into a layer having a thickness of about 10 to 15 mils, the article preferably has a Gardner toughness (failure energy) at 73xc2x0 F. (22xc2x0 C.) of at least 110 in.-lbf, and at xe2x88x9220xc2x0 F. (xe2x88x9229xc2x0 C.) of at least 100 in.-lbf. Surprisingly, the compositions exhibit improved toughness both at room temperature as well as at low temperatures.
According to another aspect of the invention, a polyester-based thermoplastic composition has a high retained viscosity following heat setting. In particular, the heat set composition preferably has a final intrinsic viscosity that is at least about 70% of the initial intrinsic viscosity of the bulk polymer. The high level of retained viscosity permits the conversion of polyesters having lower initial intrinsic viscosity, including resins that heretofore were unusable in food grade and other applications.
The continuous thermoforming process of the present invention permits the sheet to be formed without the need for tensioning or orienting, resulting in significantly improved product definition and, especially, retained product definition. It is particularly preferred that the sheet not be tensioned or oriented in either direction so as to essentially eliminate post-mold distortion. The process also improves accuracy and precision of product trimming by reducing distortion normally attendant with changes in thermally induced crystallinity as the sheet is heated and cooled during processing. As a result, the articles can be separated from the mold more rapidly following forming, which facilitates faster overall production rates.
The process of the present invention also permits more precise control of thermally induced crystallinity in the articles. The degree of crystallinity in products can be tailored to a particular application, e.g., in the manufacture of food containers such as microwave-ovenable containers, dual-ovenable containers, and the like. Significantly, the degree of crystallinity in the thermoformed article is not governed by manufacturing limitations. For example, unlike conventional continuous thermoforming devices, no minimum degree of crystallinity is required to enable the articles to be separated from the mold without distortion. Rather the degree of crystallinity in an article can be selectively controlled in accordance with a degree most suitable for a particular application.
The present invention overcomes many of the limitations associated with the prior art. For example, the present invention permits the conversion of lower melt strength materials, as well as the conversion of a variety of thermoplastic materials at a much higher rate. Polyesters having relatively low crystallinity rates and polyesters having relatively high crystallization rates can be used separately or in combination under the conditions described herein. In addition, the present invention permits the conversion of polyesters having lower initial intrinsic viscosity, including grades of resins that previously could not be used in food grade and other applications requiring high tolerances. Further, the present invention permits dissimilar materials (e.g., polar and non-polar) to be processed in co-extruded form.